Particulates or particulate matter emitted or discharged from a diesel engine is mainly constituted by carbonic soot and a soluble organic fraction (SOF) of high-boiling hydrocarbons, and contains a trace of sulfate (misty sulfuric acid fraction). In order to reduce emission of particulates from the engine, a particulate filter is conventionally employed and incorporated into an exhaust pipe through which the exhaust gas flows.
This kind of particulate filter is a porous honeycomb structure made of ceramics such as cordierite and having lattice-like compartmentalized passages; alternate ones of the passages having plugged inlets and the remaining passages having unplugged open inlets which are plugged at their outlets. Thus, only the exhaust gas passing through the compartment walls is discharged downstream.
The particulates in the exhaust gas, which are captured by and accumulated on the compartment walls of the particulate filter, are required to be burned off to regenerate the particulate filter before exhaust resistance considerably increases due to clogging. However, the exhaust from the diesel engine in a normal operating status rarely has a chance to reach a temperature level at which the particulates ignite by themselves. Then, to employ a catalytic regenerative particulate filter has been investigated which integrally carries oxidation catalyst made from platinum-carrying alumina or in which separate oxidation catalyst is arranged upstream of the particulate filter.
Thus, such employment of the catalyst regenerative particulate filter accelerates the oxidation reaction of the captured particulates and lowers ignition temperature thereof, whereby the particulates can be burned off at an exhaust temperature lower than ever before.
Other than the above-mentioned particulate filter, a post-processing device such as selective reduction or NOx-occlusion reduction catalyst for removal of NOx in exhaust gas has been proposed to be incorporated in an exhaust pipe; in particular, recently, a post-processing device in the form of a combination of a particulate filter with NOx-occlusion reduction catalyst has been developed.
However, whenever any of these post-processing devices is employed, an exhaust temperature above a predetermined level is required for assured burning-off of particulates and for obtaining enough catalytic activities. Thus, if an operating status with lower exhaust temperature (generally speaking, a region with lower exhaust temperature extends in a light-load operating region) continues, the post-processing device cannot work well and in a case of, for example, a city shuttle-bus which tends to travel on congested roads, operation above a predetermined temperature requirement does not continue for a long time, resulting in the possibility of insufficiently obtaining the exhaust emission control effect due to provision of a post-processing device.
To overcome this problem, there has been investigated arranging a plasma generator upstream of the post-processing device so as to obtain enough exhaust emission control effect due to the post-processing device even in an operating region with lower exhaust temperature. To discharge electricity by such plasma generator in the exhaust gas upstream of the post-processing device to generate plasma excites the exhaust gas and changes unburned hydrocarbon, oxygen and NO into an activated radical, ozone and NO2, respectively. Because of these exhaust gas excited components being active, exhaust emission control effect due to the post-processing device can be obtained even in a region with exhaust temperature lower than ever before.
As to an exhaust emission control device with a plasma generator arranged upstream of a particulate filter, the following Reference 1 is known as the prior art.
[Reference 1] JP 2002-276333A